1. Technical Field
The present invention relates generally to cellular wireless communication systems, and more particularly to the determination of a bit error probability of radio frequency communications received by a wireless terminal within a cellular wireless communication system.
2. Related Art
Cellular wireless communication systems support wireless communication services in many populated areas of the world. While cellular wireless communication systems were initially constructed to service voice communications, they are now called upon to support data communications as well. The demand for data communication services has exploded with the acceptance and widespread use of the Internet. While data communications have historically been serviced via wired connections, cellular wireless users now demand that their wireless units also support data communications. Many wireless subscribers now expect to be able to “surf” the Internet, access their email, and perform other data communication activities using their cellular phones, wireless personal data assistants, wirelessly linked notebook computers, and/or other wireless devices. The demand for wireless communication system data communications will only increase with time. Thus, cellular wireless communication systems are currently being created/modified to service these burgeoning data communication demands.
Cellular wireless networks include a “network infrastructure” that wirelessly communicates with wireless terminals within a respective service coverage area. The network infrastructure typically includes a plurality of base stations dispersed throughout the service coverage area, each of which supports wireless communications within a respective cell (or set of sectors). The base stations couple to base station controllers (BSCs), with each BSC serving a plurality of base stations. Each BSC couples to a mobile switching center (MSC). Each BSC also typically directly or indirectly couples to the Internet.
In operation, each base station communicates with a plurality of wireless terminals operating in its cell/sectors. A BSC coupled to the base station routes voice communications between the MSC and a serving base station. The MSC routes voice communications to another MSC or to the PSTN. Typically, BSCs route data communications between a servicing base station and a packet data network that may include or couple to the Internet. Transmissions from base stations to wireless terminals are referred to as “forward link” transmissions while transmissions from wireless terminals to base stations are referred to as “reverse link” transmissions. The volume of data transmitted on the forward link typically exceeds the volume of data transmitted on the reverse link. Such is the case because data users typically issue commands to request data from data sources, e.g., web servers, and the web servers provide the data to the wireless terminals. The great number of wireless terminals communicating with a single base station forces the need to divide the forward and reverse link transmission times amongst the various wireless terminals.
Wireless links between base stations and their serviced wireless terminals typically operate according to one (or more) of a plurality of operating standards. These operating standards define the manner in which the wireless link may be allocated, setup, serviced and torn down. One popular cellular standard is the Global System for Mobile telecommunications (GSM) standard. The GSM standard, or simply GSM, is predominant in Europe and is in use around the globe. While GSM originally serviced only voice communications, it has been modified to also service data communications. GSM General Packet Radio Service (GPRS) operations and the Enhanced Data rates for GSM (or Global) Evolution (EDGE) operations coexist with GSM by sharing the channel bandwidth, slot structure, and slot timing of the GSM standard. GPRS operations and EDGE operations may also serve as migration paths for other standards as well, e.g., IS-136 and Pacific Digital Cellular (PDC).
The GSM standard specifies communications in a time divided format (in multiple channels). The GSM standard specifies a 20 ms frame that is divided into four sub-frames, each including eight slots of approximately 625 μs in duration. Each slot corresponds to a Radio Frequency (RF) burst. A normal RF burst, used to transmit information, typically includes a left side, a midamble, and a right side. The midamble typically contain a training sequence whose exact configuration depends on modulation format used. However, other types of RF bursts are known to those skilled in the art. Each set of four bursts on the forward link carry a partial link layer data block, a full link layer data block, or multiple link layer data blocks. Also included in these four bursts is control information intended for not only the wireless terminal for which the data block is intended but for other wireless terminals as well.
GPRS and EDGE include multiple coding/puncturing schemes and multiple modulation formats, e.g., Gaussian Minimum Shift Keying (GMSK) modulation or Eight Phase Shift Keying (8PSK) modulation. Particular coding/puncturing schemes and modulation formats used at any time depend upon the quality of a servicing forward link channel, e.g., Signal-to-Noise-Ratio (SNR) or Signal-to-Interference-Ratio (SIR) of the channel, Bit Error Rate of the channel, Block Error Rate of the channel, etc. As multiple modulation formats may be used for any RF burst, wireless communication systems need the ability to determine which coding scheme and modulation format will result in the successful receipt and demodulation of the information contained within the RF burst. This decision may be further influenced by changing radio conditions and the desired quality level to be associated with the communications.
Link adaptation (LA) is a. mechanism used to adapt the channel coding schemes and modulation formats to the changing radio link conditions. LA allows the network to command the handset to change to the modulation and coding scheme that is best for the current radio condition while providing a desired level of quality associated with the communications. To facilitate LA, an accurate or representative measure of the changing radio conditions is required. The actual Bit Error Rate (BER) associated with the changing radio conditions would provide such a measure. However, exact BER evaluation is often intractable or numerically cumbersome. Therefore, approximations or probabilities of the BER are sought. Such approximations may be referred to as the Bit Error Probability (BEP). Methods used to estimate the BEP often rely on additive white-Gaussian noise (AWGN) to compute the signal to noise ratio (SNR) from which the BEP is based. Although this method is easy to apply, using the standard Gaussian approximation often overestimates system performance. This over estimation of system performance can result in optimistic BEPs being used to make LA decisions. LA decisions based upon optimistic BEP can result in lost communications between the wireless terminal and the servicing base station. Therefore a need exists to determine more accurate BEPs for use in LA decisions processes.